Update git submodules

* Update docs/submodules/yardstick from branch 'stable/danube'
  - Merge "Set work directory in Yardstick container" into stable/danube
  - Set work directory in Yardstick container

    JIRA: YARDSTICK-608

    Set the work directory to /home/opnfv/repos in yardstick docker container to
    help users better understand file structure.
    This is a improvement related with document improvement.

    Change-Id: Ief4b76e6d6898ceab4a546574f5a09d51eb0d945
    Signed-off-by: JingLu5 <lvjing5@huawei.com>
    (cherry picked from commit 343970dc9c6193e28863e4ab8bae801b5060ae6a)

diff --git a/docs/submodules/yardstick b/docs/submodules/yardstick
index f990f63..6ca8346 160000 (submodule)
--- a/docs/submodules/yardstick
+++ b/docs/submodules/yardstick
@@ -1 +1 @@
-Subproject commit f990f63a402ecd5225115c20be6bbafc8511d0db
+Subproject commit 6ca8346532caa2997588e35c66cae86a1246e9ac